Interpretive Summary: Salmonella enterica is a pathogenic bacterium that can infect both man and animals. It causes a disease called Salmonellosis which is characterized by nausea and vomiting. It is usually not life threatening to most people but in the very old, very young, and people with weakened immune systems it can be very severe and sometimes life-threatening. Although this organism can also make cows sick it is frequently able to infect cows but not cause illness. In this case it can be shed in large amounts by cows without anyone knowing and may cause contamination of milk, dairy products, and meat. In previous work we showed that Salmonella enterica serotypes Cerro and Kentucky could cause very long term infections in a dairy herd without apparent impact on herd health or milk production; we also showed that milk filters could be used to detect Salmonella infection in a dairy herd. In this study we analyzed milk filters from a number of farms in an intensely dairy-farmed region of Pennsylvania to show that infection by these two serotypes was not limited to a single farm but rather, that the infections were wide spread in the region. The regional nature of the infection is likely to complicate efforts to control these pathogens on dairy farms. This information should be very useful for other scientists, the dairy industry and regulatory agencies.

Technical Abstract:
Salmonella enterica is a zoonotic pathogen frequently associated with dairy farms. The organism can cause disease in cows but is also frequently shed in large numbers by dairy cows that are asymptomatic. Cows on a ~100 head dairy farm in Pennsylvania, USA, (focal dairy) were previously shown to have long-term asymptomatic infection with serotypes Cerro and Kentucky. Milk filters were collected from farms located around the focal dairy to determine whether the infection by Cerro and Kentucky were limited to the focal dairy or whether the infection might be more regional in nature. Analysis of milk filters showed that Cerro and Kentucky were wide-spread in the surrounding region with 16 of 39 filter sets (41%) positive for one or both serotypes. Pulsed-field gel electrophoresis showed that the Kentucky strains from the milk filters shares >90% similarity with strains from the focal dairy and from local streams. Although there was more variation between Cerro strains (>80% similarity), Cerro isolated from most milk filters were highly similar (>90%) to strains isolated from the focal dairy and local streams. In this intensely dairy-farmed region, infection of dairy cows appears to be regional in nature, a fact that will impact efforts to control these pathogens.